Pull request discussion: Adding PostgresArray type #14

I've created an extension to the library in my own code which may be useful if you'd consider a pull request. One of my data types is an array of SQL values, which there is currently no support for. The code linked here would add that type:

https://gist.github.com/kkieffer/0adadaaa351871d59d08a95d40f67cc3

The user can create an optionalArray and pull the values from it like this:

let postgresArrayValues : [PostgresValue] = try columns[8].optionalArray()?.arrayValues

Then for each value, they can extract the desired type.

The postgresValue for PostgresArray provides a raw value like this:

{"value1","value2","value3"}